1. Field of the Invention
The present invention generally relates to a light, or laser, printer in which a photosensitive or photoconductive media is moved in a first direction relative to a light, or laser, beam which is scanned in a second direction, perpendicular to the first direction, in order to selectively expose, or print, regions of the media. The present invention particularly relates to compensating for variations in the velocity of the first direction movement of the media, which velocity variations cause undesirable variations in the image produced, or printed.
2. Description of the Prior Art
In a light, or laser, printer of either the scanning head or linearly-arrayed Light Emitting Diode (LED) type, an electrically charged, photosensitive (normally a photoconductive) media is moved in front of the light source. The photosensitive media may be, for example, either a photoconductive drum or belt. This photoconductive media is selectively discharged (exposed) in certain areas by the laser beam in order to form an image. In some laser printers the toner is maintained charged at the general electrical potential of the photoconductive media, making that there is initially no attraction nor any pickup of the charged toner by the charged media. Then the photoconductive media is selectively discharged by the laser beam. Toner is thereby attracted and held only to the area of the media that is exposed to the laser beam during an electrostatic printing process, causing the printing of black (or color) in this area.
In other laser printers, the photoconductive meda and the toner are oppositely charged. An area of the media is then selectively discharged (exposed) by the laser beam during the electrostatic printing process, discharging this area to the general electrical potential of the toner. The toner is not attracted to this area of the media, thereby printing black (or color) in all other areas of the media that were not discharged (exposed) by the laser beam. The principles of the present invention are applicable to either type of laser printer. The image that is generated on the photoconductive media is transferred to a final media which may be, for example, either paper or plastic film. Alternatively, the photoconductive media may itself be a final media, such as a specially coated photoconductive paper.
In either the case of directly- or of indirectly-exposed photoconductive media, and for either the case of printing black or printing white from the laser-exposed regions, it is extremely difficult to precisely control the instantaneous velocity of the media, which is slow moving in relation to the high speeds at which it is exposed with a light beam. It is desired that this velocity should be absolutely uniform and invariant in order that the image selectively exposed on the photosensitive and photoconductive media should be correspondingly uniform and invariant. Small instantaneous velocity changes will, however, be irreducibly present in the movement of the media.
These instantaneous variations in the velocity of the moving photosensitive media have many causes. These causes are inherent in the mechanical system causing movement of the media. This mechanical system incorporates bearings, gear teeth, belts and other mechanical elements which may be subject to variation in shape, fit, finish, friction, elasticity, slip, concentricity, alignment, and other factors affecting the precision and uniformity of the mechanical drive. If an electric motor is involved then the poles of such a motor may be non-uniform. There is friction within the mechanical system between the moving and non-moving parts. The media itself may provide an irregular load on the mechanical drive system. The drive system may be subject to slip between its parts, and may be affected by shock or vibration. Finally, the entire mechanical drive system may be subject to effects of wear.
Many of the mechanical causes of slight velocity variations in the movement of the photosensitive, and photoconductive, media may be minimized by the use of precision mechanical components. However, these components (such as anti-backlash gears) significantly raise the cost of the mechanical drive system while not entirely eliminating variations in the velocity of the movement of the media.
The irreducible and inescapable velocity variations occurring in the movement of the photosensitive, and photoconductive, media within a laser printer are adverse to the quality of the printed image. These adverse effects are particularly noticeable when printing very narrow and very closely spaced parallel lines which are perpendicular to the direction in which the media is moving. Lines will appear to vary in thickness and in line-to-line spacing. They will not have the desired appearance of a finely ruled reticular grid. Additionally, the velocity variations will cause visually perceptible imperfections when a laser printer is used to print a grey scale consisting of very small black dots or squares alternating with unprinted dot or square areas. Velocity variations in the movement of the photosensitive media will appear to cause striations, or gradient variations in density, across the workpiece. These striations are formed in the direction perpendicular to the direction of media movement. Both the variations in the printing of the lines and of the grey scale may be quite small. However, the human eye is very sensitive to small changes in this type of pattern.
The visually perceptible changes in the parallel line, or grey scale, patterns primarily arise from three factors. First, there is a change in the absolute height of features being exposed and printed. This change in absolute height of the features is due to the change in velocity of the photosensitive media, and is directly proportional to such change. This change is the most tolerable to the human eye. If it were the only change occurring than both parallel black printed lines and the intervening white, or unprinted, lines would both be equally thicker or thinner with a respective slowing or speeding of the media velocity. Likewise, in a grey scale the typically square areas of black and white would each vary as respective identical black and white rectangles of heights that were either taller or shorter as the media velocity decreased or increased.
If the media velocity were to vary rapidly during the period of scanning just a few lines, or during the scanning of a single line, with the laser beam then these absolute height variations might be visually perceived as imperfections. Similarly, if the variations were to be significantly larger, then they may again be perceived as imperfections. But neither extremely rapid nor extremely large variations are normally the case. Furthermore, the human eye is typically sensitive only to the ratio between white and black areas for uniform fine patterns, and is not offended if horizontal lines, and grey-scale squares, are in some places a little "fatter" and in other places a little "thinner" so long as the ratio of black to white in these areas is constant throughout a large region, and preferably over the entire printed page. This means that a single page having thicker and thinner black horizontal lines will appear visually satisfactory (if the variation in media velocity and resultant feature height is neither too great nor too fast) so long as everywhere where the black lines are thick the intervening white lines are also thick, and everywhere the black lines are thin the white lines are also thin.
When an equal ratio of white to black is preserved everywhere in the image then the image will be perceived to be differentiated, but will also be perceived to be esthetically satisfactory. Unfortunately, the remaining two sources of visually perceptible changes in printed parallel lines, or grey scale, cause localized variations in the black to white ratio, and produce effects in the printed image, that are both readily perceived by the eye and appear as an esthetically undesirable variation in image uniformity.
The second source of visually perceptible change, and most important source of that visually perceptible change that is undesirable, in a printed image due to instantaneous velocity variations in the photoconductive media is resultant from the laser discharge (exposure) of the photoconductive media. The light intensity, and light energy, of the laser beam is essentially in a Gaussian distribution spatially around the spot where the laser light beam is focused. Meanwhile, the charged photoconductive media requires a certain amount of light energy for a certain time in order to discharge any point upon the photoconductive media to the state wherein it will print an opposite color to that which the color that a fully charged photoconductive media will print. In this selective discharge of a photoconductive media by a laser light beam there will be a threshold boundary region (relative to the focus point of the laser beam) that is dependent upon both the duration of exposure and upon the intensity level of the exposing light. This threshold boundary region discriminates between areas wherein the media has received insufficient light energy so as to remain charged, versus those areas wherein the media has received sufficient light energy during exposure to the laser beam so as to become discharged.
If the media slows down then the same amount of laser light will discharge more media area because the beam will dwell longer at each location on the media, and will expose a wider swath. In other words, if the laser beam intensity remains constant, then the effective spot size of the exposed area will increase for a slow media and will decrease for a fast media. This variation in the spot size is in both the horizontal and vertical directions. This causes a corresponding variation in the vertical size of printed features such as small horizontal lines and rectangles, and in the horizontal size of printed feature such as small verticle lines and rectangles. The precise appearance as to whether the features assume a localized appearance which is darker or lighter with diminished or increased media velocity is, of course, a function of whether the laser-exposed regions of the photoconductive media are printing white or black. However, the underlying nature of the change is the same in all instances, uniformly producing an undesirable variation in image uniformity.
A third source of visually perceptible, and undesirable, changes in a printed image due to variations in the instantaneous velocity of the photoconductive media is due to the exposure of images as a combination of scan lines. There is a lack of direct proportionality between the exposed image areas and the velocity of the photoconductive media because the exposed areas of the image are generally built up from a plurality of overlapping scan lines wherein the scanned laser beam was turned on. Since some of the change in velocity of the media results in a change in the percentage amount of overlap between successive scan lines--which overlap is not visible in the final image--then the change in the absolute height of the exposed image areas is not directly proportional to the change in media velocity. When the media is moving underneath the light, or laser, with a relatively faster speed then the exposed areas of the printed image will be of larger absolute heights, but these heights will actually be of a reduced ratio relative to those heights of the unexposed image areas, which heights are also increased. When the media is moving at a relatively slower speed then the exposed areas will be of smaller absolute heights, but these heights will actually be of an increased ratio relative to those heights of the unexposed image areas, which heights are also decreased.
An explanation of this third cause by which variations in the media velocity effect the ratio of the heights of exposed to unexposed image areas, and the equivalent ratio of the relative eights of white and black areas, is as follows. The monochrome image is created by the exposing laser beam. This laser beam so exposing a one color of the image is overlapped in its exposure of a one scan line to its exposure of the next scan line. Just one color, black or white, of the monochrome image is created by the exposing laser beam. Let this color be specified, by example and in way of illustration, to be white.
Normally an image, for example a horizontal white print line alternating with a horizontal black print line, is created by turning on the scanning laser beam for a certain number of scan lines and then, successively, leaving the scanning laser beam turned off for a certain number of scan lines. For equal width print lines, these certain numbers of scan lines upon which the laser beam is turned on and turned off are not identical. This is because there is overlap of one laser scan line to the next in the direction along the scan lines and perpendicular to the media movement. Because of this overlap a last scan line at which a laser is turned on within any white area (the white print line) will cause the white exposure of some of that height which is traced by the next scan line during which the laser is not turned on. Similarly, the last scan line at which the laser remains off at any black area (the black print line) will be partially exposed white by the next successive scan line when the laser beam is turned on.
This is simply an operation, by example, wherein white overwrites black. When successive scans of a scanning laser beam overlap, as is the actual case, then this overwrite operation immediately requires that fewer scan lines should be successively exposed, or written white, than will be next following scan lines left unexposed, or "written" black, in order to obtain equal height white and black print lines.
Moreover, this overwrite and overlap has a pronounced effect when the media varies in velocity. When the media slows then the overlap between scan lines is greater, reducing the height of the white exposed area. Also, and more importantly in visual effect, the white scan lines will overlap to a greater extent adjacent "black" scan lines, much reducing the ratio of black to white (as well as diminishing the absolute height of both features). It is this media-velocity-dependent alteration in the ratio of the heights between exposed white and black image areas which is especially detectable by, and disconcerting to, the human eye.
Drawings figures which aid in the visualization of the fairly complex second and third causes of nonuniformity in the printed image occurring with variations in the velocity of the photoconductive media will be discussed in conjunction with the description of the preferred embodiment of the present invention within this specification. For now, it is sufficient to understand that it is the maintenance of invariance in the ratio of exposed to unexposed, white to black, regions in the presence of media velocity variations that is dealt with by the present invention. If, for example, the ratio is 50%, meaning that half of the area is being printed with lines or grey scale dots or like images, then the present invention will act to preserve this 50% ratio even in the event of velocity variations in the photoconductive media. These velocity variations as would normally lead, by action of the second and third causes, to a nonuniformity in the ratio of the black and white areas within the exposed image.
After the three causes of variation in both the absolute, and the relative, heights of exposed, and printed, features dependent upon velocity of the media come to be understood, it might naturally seem (without much study or thought) that preferred solution in accordance with the present invention that will be taught within this specification is a sole, natural or only solution--especially since it works so well. This is not the case, and the solution in accordance with the present invention is actually quite exceptional in consideration of what the prior art might possibly suggest for the solution or amelioration of the image nonuniformity problem--should this problem even be recognized.
In the first place, it is not directly apparent from the prior art what could be done about this problem. Remember, the printed image nonuniformity problem particularly manifests itself as changes the ratio of dark-to-light areas in a grey scale, and as irregular heights and density of spaced parallel horizontal lines. The photoconductive media which is subject to undesirable exposure variations may be either a photoconductive drum, a photoconductive belt, or the like. These media may transfer the image to a final media such as paper or plastic film, or the final media itself may be photoconductive, such as a specially coated paper. Regardless of the processing transpiring after exposure, variations in the heights of features exposed which variations are due to instantaneous velocity variations in the photoconductive media necessarily result in undesirable, visually perceptible, variations in the printed image. Therefore, although not directly taught in the prior art, it might be reasonably hypothesized that something has got to be done to effect the areas that are exposed.
It might be firstly hypothesized that printed image variations should be attempted to be dealt with by an improved mechanical positional control of the photoconductive medium, possibly by use of an electromechanical feedback control loop. This is not the method adapted by the present invention.
It might be secondly hypothesized that variations in the uniform imaging of a photoconductive media should be dealt with by adjustment in the length of time during which such media is exposed, with a correspondingly correction to the exposed image areas. This very approach, or something similar, may indeed be considered to be that condition that fortuitously exists in non-impact electronic printers based on Light-Emitting Diode (LED) technology. In an LED printer system a linear array of LED's the width of the paper is used to write all the points, or pixels, on a given print line at one time. Instead of being based on a complex system including a single laser, a laser beam modulator, and assorted lenses and mechanical apparatus in order to sweep a laser beam positionally, the LED printer has a so-called "Light Stick" that exposes the photo conductive media surface with a single pass along its length. Each LED either lights or remains dark depending upon the bit mapped information received for that line of the image.
Of particular pertinence to the second hypothesized solution to the present problem, in the prior art LED "Light Stick" printer an encoder attached to the drive mechanism signals to the "Light Stick" print head exactly when to start exposing the media for each line. Since the LED's have a fast response time, approximately 5 micro seconds, this system provides a fairly good control of when to initiate exposure even when the media incurs velocity variations. It is, of course, necessary that the drive mechanism positional encoder should be accurate and timely to detect the precise media position regardless of velocity variations. It is further necessary that the media should not undergo significant velocity variations during movement over the smallest increment of distance, nominally one line, which is detected by the positional encoder. Both these requirements are normally satisfied.
It is not feasible to account for instantaneous variations of the photoconductive media in a laser printer employing a swept beam technology by controlling the onset time, and/or duration, of exposure. Control of the phase and the duration of exposure, which may be readily affected within an LED-array-type non-impact electronic printer, is not feasible in a swept beam laser printer. In a swept beam laser printer the laser beam is in motion under the influence of a mechanical system, and neither the onset time, or phase, of its arrival at any particular point on the image, nor its loiter, or duration, time at such point can be readily adjusted. Consequently, the present invention also rejects the approach that the phase at which exposure commences and/or the duration of such exposure should be adjusted in order to compensate for velocity variations in a photoconductive media being exposed.
Thus it seems that at least two hypotheses, each derived from consideration of the prior art, regarding how the image uniformity problem might be attempted to be solved do not, in actual fact, suggest good solutions. Remarkably, the solution in accordance with the present invention does not deal with either the mechanical movement of the media nor with the electrical phase or duration of the exposing light beam. Since many solutions, especially good ones, seem natural once they are clearly understood, it is interesting to note that the solution in accordance with the present invention is actually quite strange in consideration of anything that could be extrapolated from a direct frontal attack on the problem by a correction of the conditions of its origin, let alone by anything that could reasonably be suggested by an extension of the prior art.